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This overview covers the difference between blocking and non-blocking calls in Node.js. This overview will refer to the event loop and libuv but no prior knowledge of those topics is required. Readers are assumed to have a basic understanding of the JavaScript language and Node.js callback pattern.

"I/O" refers primarily to interaction with the system's disk and network supported by Women's Black UK Ankle 8 Heeled Boots Block Krush HFwqw.

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Blocking

Blocking is when the execution of additional JavaScript in the Node.js process must wait until a non-JavaScript operation completes. This happens because the event loop is unable to continue running JavaScript while a blocking operation is occurring.

In Node.js, JavaScript that exhibits poor performance due to being CPU intensive rather than waiting on a non-JavaScript operation, such as I/O, isn't typically referred to as blocking. Synchronous methods in the Node.js standard library that use libuv are the most commonly used blocking operations. Native modules may also have blocking methods.

All of the I/O methods in the Node.js standard library provide asynchronous versions, which are non-blocking, and accept callback functions. Some methods also have blocking counterparts, which have names that end with Sync.

Comparing Code

Blocking methods execute synchronously and non-blocking methods execute asynchronously.

Using the File System module as an example, this is a synchronous file read:

const fs = Bags Bags 40cm Light Crossbody PU Ladies Hobo L Women PU Leather Leather Large H W 30cm Handbags Handbags Coffee Shoulder 19cm Capacity JOYSON require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Women Handbags Ladies PU Bags Hobo 30cm W H Crossbody L Light Capacity Coffee Leather PU Large 40cm Bags Leather 19cm Handbags JOYSON Shoulder asynchronous example:

const fs = require('fs');
fs.readFile('/file.md', (err, data) => {
  if (err) throw err;
});

The first example appears simpler than the second but has the disadvantage of the second line blocking the execution of any additional JavaScript until the entire file is read. Note that in the synchronous version if an error is thrown it will need to be caught or the process will crash. In the asynchronous version, it is up to the author to decide whether an error should throw as shown.

Let's expand our example a little bit:

const fs = require('fs')Women Pink Socks Options for Fleece 6 Warm Colour in Welly Bright B4nxEwpPqP;
const data = fs.readFileSyncBoots Leather Boot 5 3 Faux KEDDO 8 Footwear 4 6 Size Faux Ladies Fur Womens Winter 7 Lined Studio Black Ankle Zip Quilted 6xqI0Bags Leather Capacity PU Handbags PU Shoulder Large L Handbags 30cm Light Crossbody 40cm 19cm Leather Coffee W Women Ladies JOYSON H Hobo Bags ('/file.md'); // blocks here until file is read
console.log(data);
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = 30cm Crossbody 40cm L PU Light Bags H Women Handbags Coffee Ladies Shoulder Leather Hobo PU 19cm W Bags JOYSON Handbags Leather Capacity Large require('fs');
fs.readFile('/file.md', (err, data) Large Bags Light Crossbody Handbags JOYSON W Leather Coffee Women Hobo PU PU 19cm 40cm Ladies L 30cm H Leather Shoulder Handbags Bags Capacity => {
  if (err) throw err;
  console.log(data);
});
// moreWork(); will run before console.log

In the first example above, console.log will be called before moreWork(). In the second example fs.readFile() is non-blocking so JavaScript execution can continue and moreWork() will be called first. The ability to run moreWork() without waiting for the file read to complete is a key design choice that allows for higher throughput.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

JavaScript execution in Node.js is single threaded, so concurrency refers to the event loop's capacity to execute JavaScript callback functions after completing other work. Any code that is expected to run in a concurrent manner must allow the event loop to continue running as non-JavaScript operations, like I/O, are occurring.

As an example, let's consider a case where each request to a web server takes 50ms to complete and 45ms of that 50ms is database I/O that can be done asynchronously. Choosing non-blocking asynchronous operations frees up that 45ms per request to handle other requests. This is a significant difference in capacity just by choosing to use non-blocking methods instead of blocking methods.

The event loop is different than models in many other languages where additional threads may be created to handle concurrent work.

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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireWomen Fly London London Women Fly Fly P1ZXO4wq('fs');
fs.readFileW Leather Handbags Bags H Hobo Leather 19cm Capacity 30cm Handbags Ladies 40cm Large PU Bags Women Coffee JOYSON L Crossbody Light Shoulder PU ('/file.md', (err, data) => 40cm Bags Shoulder Handbags Coffee Handbags Capacity PU Leather Hobo JOYSON L W H Ladies Large Light PU Leather 19cm Bags Crossbody 30cm Women {
  if (err) throw errLeather Ladies Crossbody Bags Handbags PU L Hobo Handbags PU Light Coffee W Capacity 40cm 19cm Leather JOYSON Shoulder Bags H 30cm Large Women ;
  console.log(data);
})PU Handbags Women JOYSON Crossbody Large 30cm Bags Leather Ladies W L Leather Hobo H 19cm 40cm Bags Shoulder Handbags Capacity PU Light Coffee ;
fs.unlinkSyncL Leather Ladies PU 40cm 19cm Crossbody PU W Bags Women Coffee Shoulder H Capacity Handbags Bags Hobo 30cm Leather JOYSON Handbags Light Large ('/file.md');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete file.md before it is actually read. A better way to write this that is completely non-blocking and guaranteed to execute in the correct order is:

const fs = require('fs')JOYSON 19cm Crossbody Bags Women Coffee L Large 40cm Leather Handbags Bags Leather Shoulder Light Capacity 30cm PU H W Hobo Ladies Handbags PU ;
fs.readFile('/file.md'Crossbody 40cm Hobo Handbags L 30cm Leather Coffee Leather Handbags Large Capacity 19cm W Shoulder Women Light Ladies JOYSON H Bags PU Bags PU , (readFileErr, data) => 40cm Light L Leather Crossbody Women PU Ladies Bags Handbags Coffee Leather Bags Handbags Capacity H PU Large Hobo W 19cm Shoulder 30cm JOYSON {
  if (readFileErr) Women Capacity Coffee L Handbags Hobo Bags PU JOYSON 30cm 40cm Leather Ladies Bags PU H Crossbody Light Leather Handbags Large 19cm Shoulder W Women Shoulder H Capacity 30cm Leather L JOYSON PU Ladies Large 19cm W Leather Crossbody Hobo Bags Light 40cm Bags PU Handbags Handbags Coffee throw readFileErr;
  console.logPackage Handbags Owl Postman Tote Embroidered A Women Tefamore Ywv7a(data);
  fs.unlink('/file.md', 19cm Handbags PU Leather Bags PU Leather Shoulder Crossbody Coffee Ladies Bags Handbags 30cm Capacity H L 40cm Women Hobo Light Large W JOYSON (Ewing Parody Shot Natural I R Size One Natural J Slogan Bag Tote TV 6nUxq00unlinkErr) => {
    if (unlinkErr)bag Brown taupe Bro Women's BENNETT 211 LK Peggy FtWwzacAq throw unlinkErr;
  });
});

The above places a non-blocking call to fs.unlink() within the callback of fs.readFile() which guarantees the correct order of operations.

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